1. Field of the Invention
The invention relates to automatic swimming pool cover systems and, in particular, to the mechanisms and devices for carrying the front edge of the pool cover across the pool above water level.
2. Description of the Prior Art
Automatic swimming pool cover systems typically include a flexible vinyl fabric which is sized so that most of it floats on the water surface of the swimming pool. The pool water acts as a low friction surface significantly reducing the amount of force required to move the cover across the pool. The front edge of the cover is secured to a rigid boom spanning the width of the pool for holding the front edge of the cover above the water as it is drawn back and forth across the pool.
To draw the cover across the pool, a cable, typically a Dacron line, is incorporated into and forms a beaded tape which is sewn or attached to the side edges of the pool cover. The beaded tape in turn is captured and slides within a "C" channel of an extruded aluminum track. The track is secured either to the pool deck or the the underside of an overhanging coping along the sides of the swimming pool. The cables extending from the beaded tape sections of the cover are trained around pulleys at the distal ends of the tracks and return in a parallel "C" channel to a drive mechanism where they wind onto cable take-up reels.
To uncover the pool, the drive mechanism rotatably drives a cover drum mounted at one end of the pool winding the pool cover around its periphery unwinding the cables from the take-up reels. To cover the pool the drive mechanism rotatably drives the cable take-up reels winding up the cables to pull the cover across the pool unwinding the cover from the cover drum.
The rate at which the pool cover unwinds from and winds onto the cover drum depends on the diameter of the roll of the cover still wound around the drum, i.e., the rate is greatest when most the cover is wound around the drum (largest diameter) and least when the cover is practically unwound from the drum (least diameter). The same phenomenon occurs as the cables wind onto and unwind from the cable reels. It should be appreciated that the cables wind onto the cable reels at the highest rate when the cover unwinds from the cover drum at its lowest rate and visa versa
In systems where the cable take-up reels and the cover drum are on the same shaft and rotate at the same rate, a spring is utilized as a tensioning take-up mechanism to compensate for the different and varying rates at which the cables and pool cover wind and unwind from the respective reels and drum during the opening and closing cycles. The spring mechanism lengthens and shortens the cable path as the cover is drawn back and forth across the pool taking up and yielding slack in the respective cables as necessary to compensate for the difference in the winding and unwinding rates of the reels and drum. [See U.S. Pat. Nos. 3,747,132 & 3,982,286, Foster.]
In other systems a clutching mechanism is utilized to decouple the rotation of the cable reels from that of the cover drum as it is rotatably driven to wind the cover onto the drum uncovering the pool, and to decouple the rotation of the cover drum from that of the cable reels as they are rotatably driven to draw the cover across the pool. Typically, in such systems, the cable reels are allowed to free wheel when the cover drum is rotatably driven and conversely, the cover drum to free wheel when the cable reels are rotatably driven. [See U.S. Pat. Nos. 3,019,450 and 3,050,743, Lamb.]
In early automatic pool cover systems the rigid boom spanning the width of the pool holding the front edge of the cover above the water was typically supported by a pair of wheeled dollies rolling on the side edges of the pool. The cables moving within the "C" channels of the track along either side of the pool were either directly secured in some fashion to the rigid boom, [Foster, supra], or were indirectly secured to the ends of the boom via fabric interfaces referred to as gores. [See U.S. Pat. No. 4,001,900, Lamb].
It should be appreciated that in such early systems debris such as towels, dirt, and leaves left at the edge of the pool, as well as irregularities in the path of the wheeled dollies, would disrupt both extension and retraction of the cover and frequently caused the rigid boom to bind either ripping the fabric interface coupling the ends of the boom to the cables or pulling the beaded side edges of the pool cover out of the track. And in instances of direct couplings between the rigid boom and cables, such binding could cause sufficient skewing to allow the boom to drop into the pool. Also folds in the cover and its beaded edge as it winds onto the cover drum during retraction would frequently cause skewing and binding of the rigid boom because of unequal winding, transversely, of the cover around the cover drum.
A factor which greatly complicates the extension and retraction of automatic pool covers across a swimming pool is the requirement for the cover to have sufficient slack both transversely and longitudinally to enable it to float on the water surface as it was extended and retracted. Accordingly, attempts have been made to configure the fabric interfaces (gores) coupling the cables to ends of the rigid boom in such a manner as to preclude transverse tension at the front corners of the cover and yet provide for longitudinal tension necessary for unwinding the cover from the cover drum during extension and for pulling the rigid boom back during retraction.
Ameliorating the complexity of stress transfer problems at the junction between the front edge of the cover, the cables and the rigid boom, is the fact that each beaded tape side edge of the cover and cable extending to the take-up reel comprise a single mechanical tensioning element that has both the capacity to carry a portion of the longitudinal tension load on the body of the cover during extension and retraction and the capacity to overcome the friction load of the beaded edges sliding within the "C" channels of the track. [See U.S. Pat. No. 4,060,860, Lamb.]
The result of the above interacting factors has been an interface fabric attached at the front corners of the cover which was designed to allow the cover to billow transversely but which maintained the longitudinal tensional integrity of the beaded edge and cable.
The primary problem with such with such fabric interfaces is that flexibility exists in the coupling between the boom and cables which allows the boom to easily skew transversely between the tracks. Such skewing unequally loads or strains the cover fabric at the front corners and causes the cover wind crookedly onto the cover drum during retraction. Also, such fabric interfaces tend to wear excessively because of the variety of tension loading experienced during extension and retraction of the cover. Also, a billowing interface fabric tends to slide and excessively wear on the coping at the edges of the pool during extension and retraction in addition to trapping debris at the front corner edges of the cover. When combined with the effects of moisture, pool chemicals and sunlight, the above factors cause such fabric interfaces to deteriorate at a faster rate than the remainder of the cover.
Other disadvantages of fabric interfaces connecting between cables cover and rigid boom related to the practical impossibility to provide a standard manufacturing design because of variations in pool widths, and variations in the distances between the level of the track and the level of the water between different swimming pools. Also shrinkage of the cover fabric due to the effects of sun, pool chemicals, and temperature complicate the design of such fabric interfaces.
Because of the problems with wheeled dollies for supporting the rigid boom spanning the pool, in 1981 the applicant began experimenting with various types of slider mechanisms which were captured by and slid in a "C" channel of the track for mechanically supporting the rigid boom spanning the pool above water level. However, tracks with separate slider channels are the exception in the pool cover industry, and track replacement is not feasible for the installed base of pool covers. The industry, following the lead of the applicant, has for the most part adopted various different types of sliding mechanisms which have today supplanted the use of wheeled dollies for supporting the rigid boom spanning the pool above the water level. (See, for example, U.S. Pat. No. 4,686,717, MacDonald et al. & U.K. Pat. No. 2,072,006, Lee.)
However, while such slider mechanisms provide an opportunity for direct connection between the ends of the rigid boom and the cables, they also introduce a host of additional mechanical problems. First, such sliding mechanisms are extremely sensitive to and can not tolerate any skewing of the rigid boom which causes the sliders to jam in the capturing track channels. Also, any forces tending to bow the rigid boom during extension and retraction will simultaneously tend to pull the sliders out of the slot opening of the capturing track channel and twist them twist horizontally causing them to jam. The tracks on either side of the pool must be exactly parallel with the spacing between them constant, otherwise the boom will bind. In short, any stress tending to bend the boom or rotate it either longitudinally, transversely or both will cause the sliders jam in the capturing channel of the track.
Typically, separate channels in the track for capturing and carrying the sliders have been suggested (MacDonald et al. & Lee, supra) to eliminate problems inherent in having to accommodate both the cable and the sliders within the same channels. And in those instances where the slider and the cables coexist in the same channel of the track, the increased wear and stress loading of capturing channel can cause irregularities in the slot opening increasing the friction resistance of the beaded tape edge of the cover captured and sliding in the channel. Such stresses can also widen the slot opening of the capturing channels sufficiently to allow the bead tape edge to slip out of the channel.
The friction load of such slider mechanisms also tends to decouple the tensional integrity of the beaded tape edges of the cover and cables extending therefrom to the take-up reels. In essence, the frictional resistance of the sliders is a variable braking force analogous to that described in U.S. Pat. No. 4,060,860, Lamb, which can not be adjusted and which can unpredictably vary. For example, during cover retraction, a sudden increase of slider friction in one channel because of rotation or bowing of the rigid boom or a rough spot in one of the capturing track channels will increase the tension along one side of the cover inducing a diagonal stress across the cover which tends to pull the respective sliders out of the slot openings of the channels further increasing the frictional resistance of the resisting slider causing it to jam in place. During cover extension, such a sudden increase of slider friction in one of the channels induces the boom to skew again tending to pull the respective sliders against the slot openings of the capturing channels causing a jam. In either case, if the drive continues to rotate the cover drum and/or cable reels, upon a slider jamming in a capturing channel, the front edge of the cover can be ripped free of the boom.
The mechanical integrity of the junction between the sliding portion of the slider captured within the track channel and the section of the slider secured to the boom is also a problem.
In short, incorporation of a slider coupling for supporting the rigid boom above the water level of the pool significantly changes the nature of the mechanical linkage between the cables, the front cover edge, the front cover corners and the rigid boom necessary achieve a smooth and uniform extension and retraction of the cover.